1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and in particular, it relates to an image forming apparatus suitable for an image bearing member of high electrostatic capacitance.
2. Description of the Related Art
In recent years, electrophotographic copying machines or the like are expected to go into the printing market in accordance with the progress of techniques in the image forming apparatus. However, in order to enter the printing market in full scale, it is an essential requirement that the quality and stabilization of images be made much higher than the present ones.
Until today, various approaches to improve image quality have been actively carried out, and among those, an approach to an image bearing member is taken up. As an important factor to decide image quality, there is an electrostatic latent image formed on the image bearing member. The electrostatic latent image is formed by decaying an exposed part on the image bearing member, which has been charged to a dark potential VD by means of a primary charger, to a bright potential VL by laser exposure.
Here, a detailed explanation will be given to a general formation process of an electrostatic latent image.
FIG. 1 is a layer construction of a general organic photoconductor (OPC) as an image bearing member.
That is, a charge generation layer 103, a charge transport layer 102, and a surface layer 101 are laminated on a support member 105 through an undercoat layer 104. The exposed light is absorbed in the charge generation layer 103 to produce charge carriers. The charge carriers thus produced are injected into the charge transport layer 2, so that they move in the charge transport layer 2 to neutralize the dark potential VD. As a result, the exposed part is decayed to the bright potential VL, whereby an electrostatic latent image is formed. In general, it is known that when the film thickness of the image bearing member is thick, the electrostatic latent image formed thereon is deteriorated. If the electrostatic latent image is deteriorated, dot reproducibility also gets worse, so it is of course impossible to obtain an image of high quality as desired.
Therefore, the thinning of the film thickness of the image bearing member is performed as one of the approaches to the image bearing member for high image or picture quality. According to the study of the inventors, it has been found that in order to achieve the dot reproducibility allowed in OPC, the film thickness should be equal to or less than 20 μm (hereinafter referred to as a thin film OPC).
On the other hand, an amorphous silicon photosensitive member (hereinafter referred to as α-Si photosensitive member) is taken up as another approach for high picture quality. FIG. 2 is a layer construction of the α-Si photosensitive member. This α-Si photosensitive member includes a charge generation layer 113, an electric charge (electron) blocking layer 112 and a surface layer 111 laminated on a support member 115 through an electric charge (hole) blocking layer 114. The α-Si photosensitive member can create the charge generation layer 113 in the vicinity of the surface layer 111, and hence it can suppress the diffusion of electric charge to a great extent, as shown in FIG. 2.
According to the study of the present inventors, it has been found that the film thickness should be 60 μm or less in order to achieve the dot reproducibility allowed in the α-Si photosensitive member. In addition, it has been found that the α-Si photosensitive member is very high in hardness as compared with the OPC, and hence has a sufficiently allowable level of durability as required in the printing market.
As described above, the thinning of the film thickness of the charge transport layer in the image bearing member and the use of the α-Si photosensitive member are picked up as approaches for high picture quality in the electrophotographic image forming apparatus. It can be said that among these approaches, the α-Si photosensitive member is capable of outputting pictures of high quality comparable to the printing level and at the same time has excellent durability as required in the printing market.
Here, note that as an image forming apparatus using an α-Si photosensitive member, there is one described in Patent Literature 1, for example.